In the oil and gas industry, the development and operation of the oil field and gas field go through several distinct phases, all of which can be affected by unwanted microbial growth or activity. Microbial contamination may occur during drilling of the well, preparing the well for production, i.e. stimulation, and production itself.
It is desirable for the efficiency and success of any oil or natural gas production operation to protect water-based fluids from microbial contamination. After a well is drilled into a subterranean geological formation that contains oil, natural gas, and water, every effort is made to maximize the production of the oil and/or gas. To increase the permeability and flow of the oil and/or gas to the surface, the drilled wells are often subjected to well stimulation. Well stimulation generally refers to several post drilling processes used to clean the wellbore, enlarge channels, and increase pore space in the interval to be injected thus making it possible for fluids to move more readily into and out of the formation. In addition, typical reservoir enhancement processes such as waterflood and/or chemical-flood need to utilize biocide as part of the waterflood and/or chemical-flood package.
A typical well or field treatment process generally includes pumping specially engineered fluids at high pressure and rate into the subterranean geological formation. The high-pressure fluid (usually water with some specialty high viscosity fluid additives) exceeds the rock strength and opens a fracture in the formation, which can extend out into the geological formation for as much as several hundred feet. Certain commonly used fracturing treatments generally comprise a carrier fluid (usually water or brine) and a polymer, which is also commonly referred to as a friction reducer. Many well stimulation fluids will further comprise a proppant. Other compositions used as fracturing fluids include water with additives, viscoelastic surfactant gels, gelled oils, crosslinkers, oxygen scavengers, and the like.
The well treatment fluid can be prepared by blending the polymer with a fluid, such as an aqueous solution. The purpose of the polymer is generally to increase the viscosity of the fracturing fluid that aids in the creation of a fracture; and to thicken the aqueous solution so that solid particles of proppant can be suspended in the solution for delivery into the fracture.
The polymers used in well treatment fluids are subjected to an environment conducive to bacterial growth and oxidative degradation. The growth of the bacteria on polymers used in such fluids can materially alter the physical characteristics of the fluids. For example, microbial activity can degrade the polymer, leading to loss of viscosity and subsequent ineffectiveness of the fluids. Fluids that are especially susceptible to microbial degradation are those that contain polysaccharide and/or synthetic polymers such as polyacrylamides, polyglycosans, carboxyalkyl ethers, and the like. In addition to microbial degradation, these polymers are susceptible to oxidative degradation in the presence of free oxygen. The degradation can be directly caused by free oxygen or mediated by microorganisms. Thus, for example, polyacrylamides are known to degrade to smaller molecular fragments in the presence of free oxygen. Because of this, biocides and oxygen scavengers are frequently added to the well treatment fluid to control microbial growth or activity and oxygen degradation, respectively. Desirably, the biocide is selected to have minimal or no interaction with any of the components in the well stimulation fluid. For example, the biocide should not affect fluid viscosity to any significant extent and should not affect the performance of oxygen scavengers contained within the fluid. The oxygen scavengers are generally derived from bisulfate salts.
Other desirable properties for the biocide may include: (a) cost effectiveness, e.g., cost per liter, cost per cubic meter treated, and cost per year; (b) safety, e.g., personnel risk assessment (for instance, toxic gases or physical contact), neutralization requirements, registration, discharge to environment, and persistence; (c) compatibility with system fluids, e.g., solubility, partition coefficient, pH, presence of hydrogen sulfide in reservoir or formation, temperature, hardness, presence of metal ions or sulfates, level of total dissolved solids; (d) compatibility with other treatment chemicals, e.g., corrosion inhibitors, scale inhibitors, demulsifiers, water clarifiers, well stimulation chemicals, and polymers; and (e) handling, e.g., corrosiveness to metals and elastomers, freeze point, thermal stability, and separation of components.
Commercially available biocides that are used to control the growth or activity of microorganisms in gas field and oil field applications include e.g. 3,5-dimethyl-1,3,5-thiadiazinane-2-thione, also named tetrahydro-3,5-dimethyl-2H-1,3,5-thiadiazine-2-thione (also commonly referred to as Dazomet or Thione), formaldehyde, glutaraldehyde and tetrakis(hydroxymethyl)phosphonium sulfate (THPS).
U.S. Pat. Nos. 7,906,463 and 7,786,054 (incorporated by reference in their entirety) disclose the use of 3,5-dimethyl-1,3,5-thiadiazinane-2-thione in gas or oil field stimulation fluids.
International Publication No. WO 2009/015089 (incorporated by reference in its entirety) discloses a biocidal mixture of glutaraldehyde and hydroxymethyl-substituted phosphorus compounds.